This invention relates to a printing or ink transfer layer, such as used in a printing device or apparatus, and more particularly to a novel ink transfer surface layer structure and its associated ink storage body. A printing member or other ink transfer surface layer, such as in ink roll, such as employed in a typewriter or register, normally has a porous ink transfer surface layer attached to an ink storage body, from which ink is supplied through the surface layer to form impressions on paper or other subject material used for printing.
The following prior methods have been employed to produce porous materials having cavities for use as an ink storage and ink transfer surface layer. (a) A method wherein after adding foaming agent to rubber containing material, the resultant material is vulcanized in a mold. (b) A method comprising the blending of salt and rubber containing material, vulcanizing the blend in a mold and then extracting the salt therefrom. (c) A method wherein the abovementioned methods are combined. (d) A method comprising the mixing of thermoplastic elastomer and salt, molding the resulting mixture in a mold by melting and extracting salt therefrom. (e) A method comprising the admixing of foaming agent and air bubbles into a plastic sol, such as polyvinyl chloride sol, and then subjecting the resultant to compressive molding under heat in a mold.
As can be seen in FIGS. 1A and 1B, the pores of a porous body produced by using prior art methods (a) through (e) have pore diameters which are not uniform and which are discontinuous. In the case of method (a) or (c), there are produced large pores a (see FIG. 1A) which are created when salt is extruded out, or when air bubbles are communicated between themselves with assistance of tiny outlet path cavities created by the salt being extruded out. In FIG. 1B, pores a' are created by foaming agent communicating mutually at their side wall portions, such as produced by methods (b), (d) and (e). In these cases, the diameter of pores a differs from that of the communicating paths b. Thus, there are generated capillaries having discontinuously changed diameters. Accordingly, ink flowing into the capillaries, has a tendency to stay in pores a, a', and mobility of the ink in the capillaries is liable to be hindered. Consequently, operation of the printing member or ink roll have an ink storage body and an ink transfer surface layer with such a structure, has to be frequently suspended during repeated printing or ink transfer. This is because the amount of ink moving from the ink storage body to the ink transfer or printing surface decreases with each repeated cycle and the amount of ink which is transfered from the ink transfer surface to the subject, such as paper, to be printed, decreases and the shade of ink on the impressed subject becomes lighter with each repeated cycle. The ink path having dissimilar and irregular diameters, hinders the smooth movement or flow of the ink.
Therefore, even though there may be a satisfactory amount of ink in the storage body, the shade of the symbol printed or quantity of ink transfered becomes lighter with each repeated printing or ink transfer cycle. Thus, it is necessary with prior art devices to wait for the ink to permeate from the storage body to the ink transfer surface after each printing impression and before the next impression is made. It is thus necessary to stop the printing or ink transfer after each impression. Efficiency and speed are thus substantially reduced.
Furthermore, until the instant invention, there was a defect or deficiency in that, upon start of printing or ink transfer, after its suspension, the shade of the character printed or quantity of ink transfered, was apt to become lighter with the increased number of times the printing or ink transfer was repeated. Also, the non-uniform mobility of the ink was found to affect the inner side part of the ink storage body so that the shade of the character printed immediately after the suspension become lighter with the number of times the printing was repeated. (See the bottom curve in FIG. 2, for an illustration of this effect).
Moreover, in the prior art, in order to regulate the shade density of the character or symbol printed or quantity of ink transfered, means were employed to harden the surface layer, or to place a layer on the back side of the surface layer to retard occurence of distortion. The latter means, disadvantageously, reduced ink mobility and hence promoted depression of shade depth of the symbol or character printed or the quantity of ink transferred.